MoS2–NiO nanocomposite for H2S sensing at room temperature

The layered 2-D materials, such as molybdenum disulfide (MoS2), are among the most promising candidates for detecting H2S gas at very low concentrations. Herein, we have designed a series of novel nanocomposites consisting of MoS2 and NiO. These materials were synthesized via a simple hydrothermal method. The microstructure and morphology of nanocomposites were studied using different characterization techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS). These nanocomposites were used as gas sensors, and the highest response (6.3) towards 10 ppm H2S was detected by the MNO-10 gas sensor among all the tested sensors. The response value (Rg/Ra) was almost three times that of pure NiO (Rg/Ra = 2). Besides, the MNO-10 sensor exposed good selectivity, short response/recovery time (50/20 s), long-term stability (28 days), reproducibility (6 cycles), and a low detection limit (2 ppm) towards H2S gas at RT. The excellent performance of MNO-10 may be attributed to some features of MoS2, such as a layered structure, higher BET surface area, higher active sites, and a synergistic effect between MoS2 and NiO. This simple fabrication sensor throws a novel idea for detecting H2S gas.


Introduction
Detecting hazardous and ammable volatile organic compounds (VOCs), such as hydrogen sulde (H 2 S), having rotten eggs smell is important for protecting human health.The excessive inhalation of H 2 S, around 250 ppm, may cause the death of human beings. 1 A very low concentration of H 2 S can cause various chronic diseases, such as poor memory, throat injury, dizziness, cough, and damage to the human nervous system. 2 In this regard, H 2 S gas sensors with high response and low limit of detection (LOD) are highly required.
Semiconductor metal oxides (SMOs) are highly recommended and studied as gas sensors because of their advantages of low cost, simple fabrication, and ability to synthesize various nanocomposites. 3Up to now, different p-type SMOs, such as nickel oxide (NiO), tin oxide (SnO 2 ), nickel cobaltite (NiCo 2 O 4 ), zinc cobaltite (ZnCo 2 O 4 ), and zinc oxide (ZnO), have received wide attention in the eld of gas sensors [4][5][6][7][8] and supercapacitors. 9Among these SMOs, cubic NiO, having a band-gap of 3.5 eV, can be used to design novel nanocomposites with different metal oxides due to its marvelous properties, such as electrical, chemical, and thermal stabilities.NiO was found to be a promising candidate for detecting various gases, such as ozone, 10 acetone, 11 H 2 S, 12 NH 3 , 13 and formaldehyde. 14However, the reported NiO gas sensors still need improvement due to some demerits, such as high temperature, low response, and poor selectivity.Designing novel nanocomposites of NiO with layered 2-D materials is crucial to achieving these objectives.
MoS 2 , an n-type semiconductor having a band-gap of 1.29 eV, is one of the most well-known 2-D transition material dichalcogenides, and especially the layered structures of 2-D materials have received considerable attention because of their strong adsorption, high reactivity, larger surface area to volume ratio, and good electrical conductivity. 15Except for the gas sensors, it has been used in many elds, such as photo-catalysis, 16 supercapacitors, 17 and lithium-ion batteries. 18Other 2-D materials also nd novel applications as gas sensors [19][20][21][22] and supercapacitors. 23However, MoS 2 can easily be composited with various SMOs to design novel electronic devices; these features make MoS 2 a very promising candidate for detecting hazardous VOCs at low concentrations.In particular, Bai et al. proposed a sensor based on the hetero-structure of MoS 2 /SnO 2 , which exposed the gas sensing properties towards NO 2 at room temperature.The optimized sensor showed a high response, short response/recovery time, good stability, and selectivity.Besides, many sensors, such as MoS 2 /SnO 2 (CO sensor), 24 CuO/ MoS 2 (NO 2 sensor), 25 MoS 2/ ZnO (NO 2 sensor), 26 Au/MoS 2 (NO 2 sensor), 27 MoS 2 /ZnO-Zn 2 SnO 4 (H 2 S sensor), 28 and PtO 2 /MoS 2 (NH 3 sensor), 29 have been exploited to investigate sub-ppm level gases.
Herein, a series of novel nanocomposites based on NiO spherical nanoparticles and layered MoS 2 were designed for detecting toxic gases.Numerous characterizations such as XRD, SEM, TEM, HRTEM, BET, and XPS were performed for these nanocomposites.The gas sensing properties of the proposed MNO-10 sensor suggested a high response, good selectivity, short response/recovery time, and reliable long-term stability towards 10 ppm H 2 S among all other tested sensors (MNO-0, MNO-5, and MNO-15).

Materials
All the chemicals used in the synthesis method were bought from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).The materials such as molybdenum disulde (MoS 2 ), nickel chloride hexahydrate (NiCl 2 $6H 2 O), and sodium hydroxide (NaOH) were utilized in the synthesis method without further purication.

Synthesis of NiO spherical nanoparticles and nanocomposites of MoS 2 -NiO
NiO spherical nanoparticles and nanocomposites with different contents of MoS 2 were subsequently synthesized via a simple hydrothermal method, and the details are expressed in Fig. 1.Initially, NiCl 2 $6H 2 O (1.5 g) was mixed with 50 mL deionized water (DI) in four different beakers; aer half an hour of stirring, various contents of MoS 2 such as 0.00 g, 0.0505 g, 0.101 g, and 0.151 g were dispersed in all four suspensions.These samples were named MNO-0, MNO-5, MNO-10, and MNO-15, respectively.Aer that, 2 M NaOH was added to adjust pH = 12.The samples were stirred for 24 h, then the next day, the mixtures were settled in 50 mL stainless steel autoclaves, and the oven was set for 24 h at the operating temperature of 180 °C.The obtained products were washed with DI and ethanol three times using centrifugation.Aer drying (12 h, 100 °C), the products were calcined at the operating temperature of 350 °C for 2 h and 2 °C min −1 .The dried samples, aer calcination, were ground in a mortar for different characterizations such as XRD, SEM, and TEM.All these tests were performed by providing various amounts of samples, such as 20-30 mg, 10 mg, 10 mg, 200 mg, and 5-20 mg powder, for XRD, SEM, TEM, BET, and XPS, respectively.

Fabrication of a sensor
The gas sensor diagram is shown in Fig. 1.The fabrication of sensors was reported in a previous work. 6Aer calcination, the paste to fabricate the sensor was prepared using 0.1 g powder and 2 drops of terpineol (the volume was approximately 0.1-0.2mL for each drop of terpineol) and ground in a mortar.Aer that, the paste was coated onto the outer surface of an alumina tube with a small brush and then heated the alumina tube in an oven for 2 h at 80 °C.The purpose of the Ni-Cr heating wire was to control the operating temperature.Pt wires were given to link the alumina tube to the gas sensor device.All the hazardous gases detected in the present work were bought from Dalian Haide Technology Company Limited.NiO is a p-type material, and the response was calculated, such as the ratio of gas sensor resistance in gas (R g ) to that of the resistance in air (R a ) (S = R g / R a ).Other important parameters, such as selectivity, stability, reproducibility, limit of detection (LOD), and response/recovery time, are discussed in this paper.

Morphology and structure
The crystalline nature of the synthesized products was observed using XRD diffraction peaks, as exposed in Fig. From SEM images, rst of all, the morphology of layered MoS 2 and spherical nanoparticles NiO were observed; secondly, it was noticed that the particle size of NiO was reduced by adding MoS 2 in MNO-5, MNO-10, and MNO-15 nanocomposites.SEM results corresponded to XRD results, which stated that by adding MoS 2 to the samples, the crystallite sizes of NiO were reduced.The uniform scattering of all the elements present in MNO-10 and EDS spectrum is disclosed in Fig. 3(j).
The decoration of NiO spherical nanoparticles onto the layered structure of MoS 2 was further veried by TEM and HRTEM images.In Fig. 4(a) and (b), TEM images of MNO-0 are disclosed, which showed the average particle size (150-200 nm) of spherical NiO nanoparticles; besides, the clear lattice fringes with a lattice spacing of d = 0.21 nm corresponded to the (200) plane of NiO nanoparticles.Also, in Fig. 4(c)-(h), TEM images proved the layered structure of MoS 2 and spherical nanoparticles of NiO, and HRTEM images exposed lattice spacing's of d = 0.62 and d = 0.21 nm related to the (002) and (311) planes of MoS 2 and NiO nanoparticles.In Fig. 4(i), the layered structure of MoS 2 was veried by TEM analysis.TEM and HRTEM revealed the particle sizes of NiO (150-200 nm).Gradually, their size was reduced by adding MoS 2 contents in the nanocomposite, corresponding to the XRD results.
In Fig. 5, the nitrogen adsorption-desorption isotherms were analyzed for the MNO-0, MNO-5, MNO-10, and MNO-15 nanocomposites to interpret their porosity and distribution.Nitrogen adsorption-desorption isotherms reveal the specic surface area of all the nanocomposites.A higher BET surface area corresponds to a higher gas-sensing response.BET surface areas of nanocomposites of MNO-0, MNO-5, MNO-10, and MNO-15, as shown in Fig. 5(a), were 62.07 m 2 g −1 , 70.23 m 2 g −1 , 82.44 m 2 g −1 and 89.52 m 2 g −1 , respectively.Besides, in the images of Fig. 5(b), the pore size distribution diagrams are seen, suggesting that all the nanocomposites have an obvious H3 hysteresis loop, which veried the presence of many pores as well as the mesoporous nature of samples. 30It was found that pore volumes and pore sizes of MNO-0, MNO-5, MNO-10, and MNO-15 were 0.2346 cm 3 g −1 , 0.2093 cm 3 g −1 , 0.3034 cm 3 g −1 , 0.2254 cm 3 g −1 , and 15.12 nm, 11.92 nm, 14.73 nm, 10.07 nm, respectively.Due to the higher BET surface areas and wide pore size distributions of nanocomposites, these were denoted as high gas sensing responsive, especially the gas sensor of MNO-10, which detected high response towards 10 ppm H 2 S.
The surface chemical composition and electronic state of MNO-0 and MNO-10 nanocomposites were measured using Xray photoelectron spectroscopy.The details are as follows: in Fig. 6(a), the full scan XPS spectrum is given, which species the presence of elements such as Ni, O, M, and S in the MNO-10 nanocomposite.Next, the high-resolution spectrum of Ni 2p is displayed in Fig. 6(b).The two peaks in the spectrum of MNO-10 at 854.9 eV and 872.8 eV corresponded to Ni 2p 3/2 and Ni 2p 1/ 2 , 31,32 along with two satellite peaks (861.2 eV and 879.7 eV).O 1s spectra in Fig. 6(c) and (d) revealed two peaks in both MNO-0 and MNO-10 nanocomposites at 529.0, 530.7 eV and 529.5, 531.1 eV, and these peaks were related to two oxygen states such as the crystal lattice (O latt. ) and chemisorbed oxygen specie (O ads.), respectively. 33In Fig. 6 235.2 eV related to S 2s, Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 and Mo 6+ 3d 5/ 2 . 34,35In Fig. 6(f), the S 2p spectrum of MNO-10 shows two peaks at 162.1 and 168.3 eV, corresponding to S 2p 3/2 and S 2p 1/2 . 36,37A satellite peak is also displayed in the spectra of S 2p.

Gas sensing performance
The operating temperature and selectivity are also one of the most important parameters for gas sensor applications.All the gas sensors based on various nanocomposites MNO-P, P = 0, 5, 10, and 15 showed responses towards 10 ppm H 2 S at different temperatures, as shown in Fig. 7.The results proved that the sensors showed the highest responses at room temperature.The sensor of MNO-10 displayed the highest response towards 10 ppm H 2 S among all other tested sensors and the response (R g /R a ) was 6.3, which was 3.15 times that of pure NiO spherical nanoparticles.The results also veried that the response was greatly affected by the operating temperature.When the temperature was increased, the electrons from the semiconductor material surface were likely to break down from the nucleus of the material and form free electrons, which in turn increased the free electron concentration and decreased the resistance as well as the response of the sensor. 3he dynamic response transients of MNO-10 towards various H 2 S concentrations are shown in Fig. 8(a).The response/ recovery times for 10, 6, 5, 4, 3, and 2 ppm were 50/20 s, 45/ 18 s, 12/16 s, 7/8 s and 6/8 s, respectively.Besides, the response to 2 ppm H 2 S was 1.8.In order to spotlight the response/ recovery speed towards 10 ppm H 2 S, we have placed a graph between resistance and res./rec.time in Fig. 8(b), which showed that when the sensor was in H 2 S atmosphere, the curve went higher (res.time, 50 s) and around (95 000 U) it was in the stable state, then in air atmosphere, it started to go back towards the original state (rec.time, 2 s).In Fig. 8(c  the response and H 2 S ppm is crucial because linearity can make a sensor a promising candidate in gas sensors.Fig. 8(d) shows that the responses of the sensor (MNO-10) were 6.3, 5.1, 4.7, 3.9, 3, 1.9 towards 10, 6, 5, 4, 3, and 2 ppm, respectively, while tting curve values were 6.7, 5.02, 4.5, 3.8, 3.2, and 2.5 based on the equation of Y = 1.7735X 0.5824 , and regression coefficient, R 2 = 0.9899.It is clear from the gure that with increasing H 2 S concentrations, the response is gradually increased, suggesting its linearity between the response and H 2 S concentrations.
Selectivity is an essential parameter for a gas sensor.In our case, the selectivity of MZCO-P, P = 0, 5, 10 and 15, based gas sensors was measured, as shown in Fig. 9(a), and the selectivity was checked out using six toxic gases such as H 2 S, C 4 H 10 , H 2 , NO 2 , SO 2 , and NH 3 (10 ppm) at RT.The sensor based on MNO-10 showed the response of 6.6, 2.0, 1.9, 2.1, 1.8 and 1.9 towards H 2 S, C 4 H 10 , H 2 , NO 2 , SO 2 and NH 3 , respectively.The selectivity of MNO-10 was calculated, such as the ratio of the highest response towards H 2 S and the second highest response towards NO 2 (selectivity = S 10 ppm H 2 S /S 10 ppm NO 2 ), and its calculated selectivity was around 3. The selectivity of results stated that the response of MNO-10-based gas sensors towards 10 ppm H 2 S was almost three times higher than other checked VOCs, which pointed out that the sensor displayed a high response and impressive selectivity towards H 2 S. In Table 1, some sensors are compared against our sensor, which suggests that the current sensor exposed high response (6.3), short response/recovery times (50/20 s), good stability (28 days), better reproducibility (6 cycles), good selectivity (3.0), and linear relationship between response and H 2 S concentration.The above features revealed that the simple fabricated sensor in our work is a potential candidate for real-time applications.As far as we know, longterm stability and reproducibility are also imperative for gas sensors.Herein, Fig. 9(b) examines the stability test for all MZCO-P, P = 0, 3, 6, 9, and 12, nanocomposite-based gas sensors, which showed that the sensors were stable for 28 days.There was no obvious uctuation in the response with time, exhibiting adequate stability for detecting H 2 S at RT.The most stable sensor was the MNO-10-based sensor among all other tested sensors.
For detecting low concentrations of VOCs at RT, high selectivity and good reproducibility are imperative for a sensor to prove its promising applicability.Considering the above factors, reproducibility is also very important.Fig. 10(a) shows the six cycles of the gas sensing performance of MNO-10.As it is clear from the gure, the resistance in gas and air atmosphere was almost the same in six cycles; aer every cycle, the resistance can go up in the H 2 S atmosphere and back into the air atmosphere, suggesting good reproducibility and reversibility of the current sensor.Fig. 10(b) shows the graph between the response of a sensor based on MNO-10 nanocomposite and relative Fig. 7 The responses of various gas sensors at different operating temperatures.humidity (RH).The sensor was tested at RT, and the response was decreased slightly at RH values of 45, 65, and 85, which veried that the sensor has impressive stability against humidity.

Gas sensing mechanism
We have synthesized a series of nanocomposites based on layered MoS 2 and spherical NiO nanoparticles and applied them as gas sensors, and the highest response towards 10 ppm H 2 S was received.For a deeper understanding, the gas sensing mechanism of nanocomposites should be studied, which is required for RT H 2 S detection.In general, the gas sensing mechanism is based on the variation of resistance in air and gas environments; 38 however, resistance changes based on the charge carrier concentration of the material surface.In Fig. 11, the gas sensing mechanism and band diagram are described to prove the sensing mechanism.In our experiments, as we can see, the main sample, MNO-10, shows the highest response towards 10 ppm H 2 S compared with other tested sensors.The highest response may be correlated to some factors explained as follows: rst of all, a p-n hetero-junction is formed between the n-type layered MoS 2 and p-type NiO, which provides a large quantity of electron transfer in the nanostructures; secondly, the layered structure can facilitate the rapid diffusion and electrons transmission density (SEM); third, the layered structure of MoS 2 enhanced the BET surface area of the nanocomposites, which can be considered one of the imperative factors to enhance the gas sensing properties such as high response, good selectivity, and short response/recovery times towards H 2 S (BET); fourth, moreover, as XPS approved it, more oxygen species can be adsorbed on the surface of the sensing material, which may help to increase the resistance.
Moreover, as shown in the diagram, when the sensor of MNO-10 was in an air atmosphere, the oxygen molecules were adsorbed on the surface of the material and accepted electrons to convert them into O 2 − (adsorbed oxygen), 39 besides making a hole accumulation layer (HAL) from the holes which are le on the surface, signifying the reduction of resistance.The reactions based on temperatures are described in eqn (1)-(4).
O 2(gas) / O 2(ads.) ,   , T > 300 ˚C Aer that, when the sensor of MNO-10 was in the H 2 S atmosphere, the sensor, herein, NiO is a p-type semiconductor.When the sensor was in the H 2 S atmosphere, it reacted with O 2 − by forming SO 2 and H 2 O, as shown in eqn (5); besides, there was an interaction between the H 2 S gas and hydroxyl species of the thin lm sensor as well.During the reaction, the electrons returned to the conduction band to recombine holes simultaneously, which decreased the thickness of HAL, increasing the material's resistance.This would improve the gas sensing properties of the current sensor, and MNO-10 showed a p-type behaviour.

Conclusions
In this study, we designed a new series of sensors based on the nanocomposite of layered MoS 2 and spherical NiO nanoparticles; these nanocomposites were synthesized hydrothermally.Various characterizations were performed, for example, XRD, SEM, TEM, HRTEM, BET, and XPS, to conrm morphology and structural properties.From XRD, it was shown that the crystallite size was reduced with the increase of MoS 2 contents in the nanocomposites, and particle size also decreased by adding MoS 2 (SEM, TEM).Further, BET results proved that the nanocomposites have higher BET-specic surface areas than pure NiO and more oxygen  adsorption of MNO-10 than pure NiO veried by XPS.Aer that, the synthesized materials were tested as gas sensors, and the gas sensor was fabricated through a mixture of solid and terpineol oil.Various sensors based on nanocomposites (MNO-A, A = 0, 5, 10, 15) were used for detecting four kinds of toxic gases.The highest response towards 10 ppm H 2 S gas was detected by the gas sensor of MNO-10 among all other tested sensors.The response was 6.3 for 10 ppm H 2 S.Besides the highest response, the sensor of MNO-10 exposed good selectivity (3.0), better stability (28 days), minimum LOD (2 ppm), good reproducibility (6 cycles), and an almost linear relationship between response and concentrations of H 2 S (2-10 ppm).The marvelous gas sensing properties of MNO-10 were related to some crucial points such as p-n hetero-junction, layered structure of MoS 2 , higher BET surface area, and increased adsorption of oxygen species.Room temperature H 2 S gas sensors fabricated through simple ways are nding novel applications in daily routine activities.In this way, our synthesized nanocomposite-based gas sensor has promising applicability.

Conflicts of interest
This work does not have any conict of interest.

2 .
The diffraction peaks observed from the patterns corresponded to the crystallographic standard database les of MoS 2 and NiO.The XRD diffraction patterns of all the samples showed the diffraction peaks of NiO at the 2q values of 37.24°, 43.27°, 62.87°, and 75.41°, corresponding to (111), (200), (220), and (311) planes of NiO (JCPDS no.47-1049), respectively.Except for the diffraction peaks of MNO-0, one high-intensity MoS 2 peak and two small peaks were observed in all other nanocomposites at 2q values of 14.37°, 39.53°, and 49.78°, which matched with (002), (103) and (105) planes of MoS 2 (JCPDS no.37-1492).No extra peaks were found in all patterns, suggesting that the samples were well-ordered and possessed good crystallinity and clarity.Besides, crystallite sizes of NiO spherical nanoparticles in all nanocomposites were calculated using the Debye-Scherrer formula, yielding 15.59, 10.86, 9.36, and 10.07 nm based on the (200) peaks, while 14.02, 10.23, 9.98, and 9.39 nm based on the (111) peak in the nanocomposites of MNO-0, MNO-5, MNO-10, and MNO-15, respectively.It was found that adding MoS 2 in the nanocomposites decreased the crystallite size of NiO.Fig. 3 reveals the SEM images of all the nanocomposites.The spherical NiO nanoparticles with an average particle size of 150-200 nm are seen from the SEM images of MNO-0 in Fig. 3(a) and (b).The SEM images of the nanocomposite MNO-5 in Fig. 3(c) and (d) and SEM images of the nanocomposite MNO-10 and MNO-15 are displayed in Fig. 3(e)-(h), respectively.Fig. 3(i) shows the SEM image of MoS 2 displaying the layered structure.

Fig. 8
Fig. 8 (a) Dynamic res./rec.curve and resistance of MNO-10 to 10-0.5 ppm H 2 S at RT, (b) resistance change of MNO-10 composite-based sensor to 10 ppm H 2 S, (c) the plots of response/recovery times towards 10 ppm C 4 H 10 , H 2 , and NO 2 of MNO-10, (d) relation between response and different concentrations of H 2 S of MNO-10.

Fig. 9
Fig. 9 (a) The selectivity test for all sensors towards various gases at RT, (b) the stability test for all sensors at RT.

Fig. 10 (
Fig. 10 (a) The reproducibility of MNO-10 based gas sensor at RT towards 10 ppm H 2 S, (b) the response vs. different RH at RT towards 10 ppm H 2 S.

Fig. 11
Fig.11The gas sensing mechanism and energy band-gap structure of MNO-10 based gas sensor at RT towards 10 ppm H 2 S.